Applying voltage to a DC motor will cause heat corresponding to coil resistance and a coil temperature rise. Excessive coil temperature rise can damage a coil coating, and can cause a harmful influence such a short circuit and generation of heat. Accordingly, it is necessary to limit the voltage application before an excessively high temperature occurs.
Thus, conventionally, there has been a device that measures the coil temperature with a temperature sensor.
In addition, there has been a device that estimates the relative temperature change of a motor by calculating the average or integral of the product of the terminal voltage of the motor and the motor current using a phenomenon that the coil resistance of the motor increases with the temperature rise (for example, see Patent Document 1).
In addition, there has been a device that estimates the coil resistance from the motor current, bus voltage and PWM (Pulse Width Modulation) time width, and calculates the coil temperature using relationships between the coil resistance and coil temperature obtained in advance (for example, see Patent Document 2).